Beilstein J. Org. Chem. 2013, 9, 2803–2811.
shifts (δ) are given in parts per million (ppm) using the solvent phases were combined. After drying over magnesium sulfate
peak as an internal standard. Fourier transform infrared (FTIR) and filtration thereof methylene chloride was distilled off under
spectra were recorded on a Nicolet 6700 FTIR spectrometer reduced pressure and the product dried in vacuo. 11.9 g
equipped with an ATR unit. Turbidity measurements were (62.9 mmol = 87.8%) of a colorless oil was obtained. FTIR
conducted using a Tepper TP1 cloud-point photometer (diamond) ν (cm−1): 3500–3200 (br, NH), 1753 (s, ester), 1695
equipped with a laser with a wavelength of 670 nm. For all (vs, carbamate); 1H NMR (300 MHz, CDCl3) δ 5.22 (s, 1H,
experiments a heating or cooling rate of 1 °C/min during contin- NH), 3.78 (d, J = 5.9 Hz, 2H, CH2), 3.62 (s, 3H, OCH3), 1.33
uous stirring was set and the critical temperature was deter- (s, 9H, RC(CH3)3); 13C NMR (75 MHz, CDCl3) δ 170.5 (1C,
mined at 50% of relative transmittance. Oscillatory rheological R-C(O)-OR), 155.4 (1C, RNH-C(O)-OR), 79.4 (1C, RO-C-
measurements were performed on a Haake Mars II rheometer Me3), 51.7 (1C, OCH3), 41.8 (1C, CH2), 27.9 (3C, RC(CH3)3);
by ThermoFisher Scientific. For this purpose a plate–plate ESIMS (acetone) m/z: 190.1 [M + H]+, 212.1 [M + Na]+.
construction (MP35, PP35Ti) was used. The temperature was
set to 25 °C and was determined in the measuring plate with an Synthesis of 4: 9.39 g (49.7 mmol) of 2 were mixed with
accuracy of ±0.1 °C. The thermostat was regulated directly by 11.21 g (189.7 mmol) isopropylamine (3) and 5.02 g
software. For each sample the measurement period was set to (49.7 mmol) triethylamine. The mixture was stirred at room
3
0 h. Sol–gel-transmission was determined by the arithmetic temperature for 3 days and subsequently refluxed for 2 days. In
average of the first three intersections of G’ and G’’ taking an adjacent step the solution was concentrated and precipitated
initial fluctuations due to homogenization and sheer-thinning in 500 mL of n-hexane. After drying in vacuo 8.6 g (39.8 mmol
processes not into account. Glass transition temperatures (Tg) = 80.1%) of colorless crystals (melting point 102 °C) were
were determined using a Mettler Toledo DSC 822e equipped obtained. FTIR (diamond) ν (cm−1): 3292, 3222 (br, NH), 1692
with a sample robot TSO801RO. For calibration, standard tin, (vs, carbamate), 1657, 1548 (vs, amide); 1H NMR (300 MHz,
indium, and zinc samples were used. Heating and cooling CDCl3) δ 6.41 (s, 1H, RC(O)-NH-R), 5.57 (s, 1H, R-NH-
curves were determined between −30 and 50 °C or −30 and C(O)OR), 3.98 (m, 1H, CH-Me2), 3.66 (d, J = 5.7 Hz, 2H,
6
0 °C at a heating rate of 15 °C/min. The Tg values were calcu- CH2), 1.36 (s, 9H, RC(CH3)3), 1.06 (d, J = 6.6 Hz, 6H,
lated from the arithmetic average of the inflection points of the RHC(CH3)2); 13C NMR (75 MHz, CDCl3) δ 169.1 (1C,
second, third and fourth heating curve. Electrospray ionization R-C(O)-NHR), 156.7 (1C, RHN-C(O)-OR), 80.4 (1C, C-Me3),
mass spectrometry (ESIMS) was conducted on a Bruker maXis 44.9 (1C, CHMe2), 41.9 (1C, RHN-CH2-C(O)R), 28.8 (3C,
4
G mass spectrometer and matrix-assisted laser desorption- RC(CH3)3), 23.1 (2C, RHC(CH3)2); ESIMS (acetone) m/z:
ionization time-of-flight mass spectrometry (MALDI-TOF-MS) 217.2 [M + H]+, 239.1 [M + Na]+.
on a Bruker Dalomics Ultraflex 1 mass spectrometer. Melting
points were obtained using a Büchi Melting Point B-545 appa- Synthesis of 5: 7.56 g (35.0 mmol) of 4 were suspended in
ratus at a heating rate of 1 °C/min. Dynamic light scattering 40 mL of water. On heavy stirring 30 mL of concentrated
(
DLS) experiments were conducted on a Malvern Zetasizer hydrochloric acid was added dropwise. The now homogenous
Nano ZS ZEN 3600 at a temperature of 20 °C with a laser solution was refluxed for 3 h and stirred at room temperature
wavelength of 633 nm and a detection angle of 173°. The non- overnight. Afterwards, a pH value of 12–13 of the solution was
negative-least-squares algorithm was used for interpretation. adjusted by the addition of sodium hydroxide. The solution was
The samples were dissolved in pure water of pH 7 and in a concentrated and subsequently lyophilized. After extracting
concentration of 1.5 mg/mL. Each measurement was performed with methylene chloride the solvent was distilled off under
at least five times and number-averaged diameters were used as reduced pressure and the resulting residue was dried in vacuo to
result.
give 1.14 g (9.8 mmol = 28.1%) of a colorless oil, that turns
slightly yellow on heating or after time. FTIR (diamond), ν
Synthesis of 2 [21]: 8.99 g (71.6 mmol) of glycine methyl ester (cm−1): 3600–3000 (br, NH, NH2), 1644, 1544 (vs, amide); 1H
hydrochloride (1) were suspended in 60 mL methylene chloride NMR (300 MHz, CDCl3) δ 7.08 (s, 1H, NH), 3.82 (m, 1H, CH),
and 6.05 g (72.0 mmol) sodium hydrogen carbonate dissolved 3.05 (s, 2H, CH2), 1.49 (s, 2H, NH2), 0.93 (d, J = 6.6 Hz, 6H,
in 80 mL water and 12 g (205.3 mmol) sodium chloride were RHC(CH3)2); 13C NMR (75 MHz, CDCl3) δ 171.3 (1C, C(O)),
added. Under strong stirring a solution of 15.19 g (69.5 mmol) 44.5 (1C, CH2), 41.0 (1C, CH), 22.8 (2C, RHC(CH3)2); ESIMS
di-tert-butyl dicarbonate in 40 mL of methylene chloride was (acetone) m/z: 117.1 [M + H]+.
poured slowly into the mixture. Subsequently, the reaction
batch was refluxed for 3 h and stirred at room temperature Synthesis of 7 [22]: A mixture of 20 g (109.5 mmol) L-lysine
overnight. The phases were separated, the aqueous phase was monohydrochloride (6), 4.38 g (109.5 mmol) sodium
washed twice with 30 mL of methylene chloride and the organic hydroxide, 60 g (589 mmol) aluminium oxide and 300 mL
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